Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application, or patent cited in this text is not repeated in this text is merely for reasons of conciseness.
The following discussion of the background to the invention is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge of the person skilled in the art in any jurisdiction as at the priority date of the invention.
RFID involves readers (also referred to as interrogators) and tags (also referred to as cards or labels). RFID tags are devices operable to send data such as, for example, an identification (a “tag ID”) to an RFID reader for identification purposes.
In operation, a reader will attempt to communicate with one or more tags within the reader's transmission area or field. The reader is operable to transmit a predetermined signal (in the transmission area or field) and then monitors the signal. Tag(s) responding to the signal are operable to modulate it in a predetermined manner which is identified by the reader.
FIG. 1 of the drawings depicts a conventional arrangement of an RFID system 10 comprising a typical low frequency (125 KHz) RFID reader 12 having a tuned loop reader antenna 14 operable to receive a response signal from a typical tag 16. FIGS. 2A and 2B of the drawings depict simplified block diagrams for the reader 12 and the tag 16, respectively.
The reader 12 comprises a reader microprocessor 18 operable to provide a stable 125 KHz reference frequency from an onboard pulse-width modulation (“PWM”) output. This is amplified by an un-modulated RF reader amplifier 20 and used to power the reader antenna 14 at a frequency of 125 KHz. Current in the loop of the reader antenna 14 generates an inductive alternating current (“AC”) field around the loop. Also connected to the loop of the reader antenna 14 is an envelope detector 22, the simplest of which may have the form of a diode detector. Output from the envelope detector 22 is presented to a detector amplifier 24. This is depicted in FIG. 2A as being operably connected or going to an analogue-to-digital converter (“ADC”) 26, but its output could be taken to a comparator in simple readers. The components of the reader 12 are operably connected such that any signal modulation that appears on the tuned loop of the reader antenna 14 will be detected and amplified.
The tag 16 comprises a tuned loop tag antenna 28 operably coupled to a tuned circuit. A tag rectifier 30 is provided and is operable to tap off some of the power in the tuned circuit to power or run a tag microchip 32. The tag 16 further comprises a clock extractor 34 operable to divide the RF frequency by a factor, which may be, for example, 32, to provide an output data rate, a 64 bit shift register 36 containing the tag data, and a tag modulator 38 operable to modulate the tuned loop of the tag antenna 28. When the tag 16 is placed in an RF field (such as the transmission area or field) generated by the reader 12, a voltage on the tuned circuit of the tag 16 increases or builds up until the tag rectifier 30 is operable to supply enough power for the tag microchip 32 to work or function, that is energise the tag 16. A typical tag will have 64 bits of stored data in the shift register 36, although there are many different tags available with memories storing varied amounts of data from just a few bits to many thousands of bits. For a better understanding, the tag may use the clock extractor 34 to divide down the 125 KHz frequency by 32 and use this as a reference frequency. Typically this reference frequency can be used as a clock to rotate the shift register 36 containing the tag data, such as the tag ID. The shift register 36 is arranged to rotate the 64 bits of data around and around in a continuous loop. A serial output of the shift register 36 is used to modulate RF voltage on the receiver coil of the tag 16. The data is usually converted into Manchester or Bi-phase encoding to ensure that the signal has no direct current (“DC”) component. A typical waveform in this regard is depicted in FIG. 3 of the drawings.
The tuned loop of the tag antenna 28 is coupled into the tuned loop of the reader antenna 14 such that the modulation of the tag 16 also appears on the tuned loop of the reader antenna. This modulated signal can be several tens of μV to several tens of mV depending on the distance between the tag 16 and the reader 12. By operation of the envelope detector 22 and detector amplifier 24 of the reader 12, the signal is detected and amplified and presented to the ADC 26 and then subsequently to the microprocessor 18. Many microprocessors have internal ADCs. In a normal or traditional (non-anti-collision system) tag reader, the analogue to digital is used to detect when the signal is positive or negative compared to a no signal voltage, allowing for the received tag data to be decoded back, from the encoded Manchester code, for example, to raw data. Many readers also use a standard integrated circuit (“IC”) comparator in this position and present the output to a microprocessor port for decoding and processing.
Often the detected signal is amplified until it limits, rail to rail, and this can make detection easier. Typically the received waveform can be compared to a centre rest voltage with a comparator or digitally using an ADC and subtracting samples. The timings between switching are compared and the associated bit, ‘0’ or ‘1’, chosen that corresponds best to the particular encoding of the tag data.
As a general rule, an RFID system such as that described above works well. Cards or tags and readers are typically inexpensive and to date, this system is the widest in use of all card/tag systems and is used for many applications, including asset tracking, door entry, logistics, and maintenance.
However, this setup will fail if several cards or tags are in the field of the reader at the same time because modulations are overlaid and corrupted and possibly no card will be read at all.
If there are several tags in the field but one tag has more modulation than the others combined then the data of this, the dominant tag, may still be read by the reader because the other tags only have a limited influence. Such a case is depicted in FIG. 4 of the drawings. In the case depicted, a second tag B is in a field of a reader with another tag A of lesser modulation. It can be seen that using a comparator with one input set to a mid way voltage and the other connected to the output of a signal amplifier the comparator will give an output exactly according to Tag B. Once again, the timings between the switching points can be compared with the timings for encodings for ‘0’ and ‘1’ and the appropriate data bit selected. The data of tag A will not be read. In this example only a single tag can be read. However, if there are, for example, three tags contributing signal strengths having magnitudes of say 4, 3 and 2, then despite the fact that the tag with the contribution of 4 is making a larger signal contribution than any other single tag, its signal contribution is still smaller than the combined contributions of the other two tags (3+2) and none of the three tags will be able to be read by the system. The system may also break down, that is fail to make a reading, if there are two tags of equal signal contribution because the data can cancel, as depicted in FIG. 9 of the drawings. This system can have the advantage, at least for a single tag, of having long read range.
A reader employing an ADC would do the same thing in digital form by comparing the samples with the centre voltage. The higher the sample rate the better, but a sample rate of 5 times the data rate will be adequate. Thus, such a system may read data from a dominant card in a plurality of cards in the reading field, and that is all.
Substantial efforts have been made to create special cards that by various means allow the data to be gathered from each card without corruption where there is a plurality of cards present in a reading field. Such cards are known as anti-collision cards.
One system addresses this problem by using a pseudo-random timing sequence in the logic of the tag so that the tag only turns on for a short time and then turns off, so that if several tags are in the same field simultaneously, the likelihood is that only one of them will be sending data while the others are disabled. In this and other similar systems it is only possible to read the data of a particular tag if it alone is the only tag that is actively transmitting. In another system, described in International Patent Application Publication No. WO/1999/067735, the cards are read by the reader using a binary search so that cards drop out if their coding is different to the coding sent out by the reader (WO/1999/067735). Major disadvantages of these and similar systems are high cost and high complexity.
Prior art anti-collision RFID systems require the reader to communicate with the tags. This is usually accomplished by turning the RF field generated by the reader on and off and the cards detect this. Many different commands can be sent to the tags as a series of ‘0’s and ‘1’s. Typically a reader will send a ‘0’ to the tag as a longer or a shorter off pulse than the off pulse for a ‘1’. The timing of the off period for a ‘1’ and the off period for a ‘0’ are predetermined. It is important to note that the turning of the RF field on and off is solely to send commands or communicate with the tags. There is no other reason for the reader to turn the RF power on and off.
An anti-collision tag is a tag that has special provision to enable it to be read when several tags are in the same field at the same time.
A normal or conventional tag is an RFID tag that has no provision for anti-collision
It is against this background that the present invention has been developed.